The present invention relates to a washing machine, in particular a laundry washing or washing/drying machine or a dishwasher, equipped with a device for detecting the temperature of the electric resistance used for heating up the wash liquid. Washing and washing/drying machines are usually provided with a tub that houses a rotary drum in which the laundry to be washed is placed; the tub is filled with water taken from the water mains, to which washing agents such as detergents or softeners are added.
In order to improve the washing performance, a known solution provides for raising the temperature of the wash liquid (i.e. water and any washing agents) to values which may vary between approximately 30° C. and 90° C., depending on the desired type of wash; this temperature increase is obtained through an electric resistance secured to a wall of the tub, which resistance is turned on when it is submerged in the wash liquid (i.e. water only or water and washing agents) so as to exchange heat with the latter and bring it to the desired temperature, which is detected by a dedicated temperature sensor fitted within the tub.
Dishwashers are equipped with a tub that houses containers in which the crockery to be washed is placed; in this case as well, the wash liquid is heated by an electric resistance located on the tub bottom.
It is known that such electric resistances comprise a metal filament (which becomes hot by Joule effect when current is flowing through it) and a protective external covering, or “shield”, adapted to insulate the filament electrically from the surrounding environment. For the purpose of preventing the resistance from overheating, devices have been developed for detecting a resistance threshold temperature and switching the resistance off once it has reached said temperature threshold, thus avoiding the risk of damage to the resistance itself, to the machine or to the load (laundry or crockery, according to the case) as well as the risk of a short circuit or a fire.
By “overheating temperature” it is generally meant the temperature at which the resistance gets damaged from heat; this temperature may vary according to the case as a function of the construction parameters of the resistance itself; in general, the temperature at which a washing machine's resistance is damaged by heat is approximately 800° C., but the temperature threshold set for triggering the devices adapted to switch off the power supply is normally lower in order to prevent any damage to the tub, which is often made of plastic.
Some of these devices detect the resistance temperature indirectly, e.g. by using, as a sensor, an electric fuse arranged in series with the electric resistance, so that when the current drawn by the resistance reaches a certain value, the fuse will melt and break the power supply to the resistance.
Though simple and effective, this solution does not allow to keep the temperature reached by the resistance directly under control: as a matter of fact, the melting of the fuse occurs as a function of the heat generated by Joule effect by the current flowing through it, and for this very reason the resistance temperature reading may in some cases be inaccurate.
Aiming at obtaining a more accurate detection of the resistance threshold temperature, other devices have been conceived which comprise a temperature sensor capable of detecting when said temperature is reached.
Some of these devices comprise microswitches controlled by a metal rod which expands when heated, thus opening the power supply circuit as soon as the resistance temperature reaches a threshold value.
Other devices are more generally fitted with a thermostatic switch that detects when the resistance reaches its threshold temperature, at which point it will break the power supply circuit.
Yet another system has been described in the European patent application published under number EP 0 579 170 in the name of CEBI S.p.A., wherein the power supply is switched off by means of an elastic element kept in position by a support adapted to melt as soon as the resistance threshold temperature is reached.
In general, washing machines are based on the principle of using devices for detecting the achievement of the threshold temperature and preventing the resistance from overheating, which devices comprise a sensing element specifically dedicated to that task and arranged in a watertight region of the machine, outside the tub.
Furthermore, although these devices perform their function correctly, it is nonetheless necessary to provide a dedicated housing in the machine and to prearrange the machine with a circuit specifically intended for this purpose.
When the sensor comprises elements adapted to melt under the action of heat, this implies a further drawback: in fact, every time the resistance reaches its threshold temperature it will be necessary to restore the electric continuity by replacing the melted part, which is a relatively long and difficult task.
The problem of the electric resistance getting overheated is also particularly felt when the machine includes a steam treatment cycle wherein steam is produced by means of the very same electric resistance housed in the tub, as described, for example, in the European patent EP1275767 to V-Zug AG; in such a case, in fact, the production of steam causes a proportional lowering of the level of the water in the tub, with a high risk that the resistance is left emerged from it.
Another known problem suffered by washing machines, in particular laundry washing and washing/drying machines or dishwashers, is that calcareous deposits accumulate on the surface of the resistance, thus reducing the latter's efficiency to a point where it may need to be replaced as a whole unit.
As known, scale is formed from deposited calcium and magnesium being present in the water supplied by the water mains, and creates a kind of “sleeve” or tubular coating that envelops the resistance.
In order to reduce the phenomenon of calcareous deposits, an anti-scale chemical product capable of reducing said deposits is typically added to the washing agents.
As an alternative, resistances are used whose external covering is made of ceramic and it is suitable for reducing the deposits of calcium and magnesium.
These solutions prove to be effective in reducing the speed at which calcium and magnesium are deposited; however, they are not definitive solutions, since it is common practice to periodically remove the deposits which have nevertheless accumulated or even replace the whole resistance, according to the circumstances. Also, it should not be overlooked that the addition of an anti-scale product to the washing agent in the former case, and the use of a special resistance being more expensive to manufacture in the latter case, imply drawbacks from an economical viewpoint.